1. Field of the Invention
The present invention relates to a probe used for surface enhanced vibrational spectroscopic analysis and a method of manufacturing the probe.
2. Description of the Related Art
When a sample is irradiated with laser light, Raman scattered light, which is different in frequency from original incident light, is emitted from the sample together with Rayleigh scattered light, which is equal in frequency to the original incident light. The Raman spectroscopic analysis method of analyzing the Raman scattered light is effective to determine a molecular structure or a bonding state of crystals.
However, there is a case where a sample such as an organic substance is sensitive to damage by laser light, so it is necessary to measure the sample at minimum laser intensity. Because the intensity of the Raman scattered light is extremely weak, when the sample is a thin film or when a measurement area is very small, it may be difficult to obtain the Raman spectrum. Therefore, there is required a technique for detecting the Raman scattered light whose intensity is extremely weak at high sensitivity even when the sample is irradiated with laser light at an intensity level at which the sample is not damaged.
An example of the technique includes surface enhanced Raman scattering (SERS) (see Chem. Phys. Lett., Vol. 126, p.163 (1974)). The SERS is a phenomenon in which the intensity of Raman scattered light from a sample of a monomolecular layer or a several-molecular layer which is deposited on a substrate on which a metal film made of a noble metal such as silver, gold, or copper (island-like film or fine particle film) is formed becomes 102 to 106 times larger than the intensity of Raman scattered light from a sample deposited on a substrate on which the metal film is not formed. It is also necessary to make the surface of the metal film rough. For example, a film which contains Si particles, Ag particles, or CaF2 having a μm size is formed as a base film. When the metal film is formed on the base film, the roughness of the surface of the metal film increases, so the SERS is observed at higher sensitivity (see J. Phys. Chem. 1985, 89, 5174-5178, and Solid State Communications, Vol. 55, No. 12, pp. 1085-1088, 1985). Even when the metal film is deposited on the surface of the sample, the SERS phenomenon is observed.
The same is expected even in the case of an infrared spectroscopic analysis method. When the sample is irradiated with infrared light, an infrared light having a frequency peculiar to the sample is absorbed thereinto. Information with respect to molecular structure or environments of the molecules is obtained based on a frequency at an absorption position.
In recent years, a scanning probe microscope, a near-field microscope, and an atomic force microscope have been under development. Therefore, the structure of each metal nano fine particle can be measured in the nanoscale and simultaneously the interparticle distance can be controlled to detect Raman scattered light only from a specific particle to which an extremely small amount of molecules are absorbed. For example, according to J. Phys. Chem. B. 2003, 107, 7607-7617, it has been reported that when a metal nano structure which produces sufficient SERS is irradiated with laser light, the local electric field intensity on the surface of the nano structure is obtained by numeral calculation, thereby finding the metal nano structure which provides a very large enhanced intensity. When a local electric field intensity on an isolated spherical or elliptical metal nano particle is calculated, only an SERS enhanced intensity of 104 to 105 is obtained. In contrast to this, an enhanced intensity of 1010 or more, which is equivalent to the monomolecular sensitivity, is obtained on a bonding area between spherical or elliptical nano particles at an optimum wavelength without depending on the particle size. Therefore, it is reported that the very large enhanced intensity of the monomolecular sensitivity is obtained on the aggregation of the metal nano particles and the bonding area therebetween.
A metal is filled into fine pores and exposed to shorten the distance between respective exposed metal fine particles to several nm. A sample to be analyzed is attached to the surfaces of the exposed metal fine particles and irradiated with laser light. Therefore, surface enhanced Raman scattering measurement using an electromagnetic field generated between the metal fine particles to improve the sensitivity can be performed (see U.S. Publication 2005/0108085).
When a tip portion of a near-field microscope probe such as a probe in which a metal film whose thickness is several nm to several tens nm is formed on the surface thereof is irradiated with light, a locally strong electromagnetic field generates near the tip portion. When the tip portion of the probe is irradiated with light while the tip portion of the probe is brought close to or into contact with a measurement sample, the surface enhanced Raman scattering measurement is performed at high sensitivity. Such a phenomenon is called tip enhanced Raman scattering (TERS) (see Chemical Physics Letters, 335, 369-374, 2001). According to this report, for example, when the probe coated with the metal film is brought close to or into contact with the measurement sample, a sufficient intensity is obtained because of SERS.
In particular, in order to obtain a sufficient SERS intensity using metal nano structures such as particles, it is necessary to arrange the metal nano structures at an interval of approximately 0 nm to several nm. According to the conventional tip enhanced Raman scattering technique, the film thickness of the metal film applied as a coating of the tip portion of the probe can be controlled. However, it is difficult to control a shape of the metal film (to obtain island-like film or particle-like film). In addition, it is difficult to control the interval between metal fine particles.
Molecules of the measurement sample may be absorbed to the metal film coating the tip portion of the probe every time the probe is brought into contact with the measurement sample. Therefore, there is also such a problem that the probe is frequently discarded.